1. Field of the Present Description
The present invention relates to seals. In particular, the present invention relates to seals useful in controlling the flow of exhaust gas exiting from a jet engine.
2. Description of Related Art
Many types of aircraft use turbines to provide the power necessary for flight. One example would be a rotary wing aircraft with a turbine driving the rotating wing. In such aircraft the turbine engine is securely fastened to the airframe of the rotary aircraft and an exhaust system may be attached to the exhaust end of the engine to redirect exhaust gases as desired. So long as the exhaust system is relatively lightweight, the engine can support the extra load. Because the exhaust system is attached directly to the engine the seal between the exhaust system and the engine is relatively simple. The main concern at this joint is the support of the exhaust system.
Recent advancements in exhaust systems have led to heavier exhaust systems that reduce the heat signature of the aircraft as viewed through infrared equipment, among other advantages. Such exhaust systems make the aircraft more difficult to spot and follow with infrared equipment, which is very important in military applications.
Due to the added weight of the infrared suppressing exhaust system, the exhaust system is no longer light enough to attach to the engine for support. Instead, the exhaust system must be mounted directly to the airframe. Because the engine and the exhaust are mounted to different parts of the airframe, and because airframes flex during use, the exhaust and the engine are no longer relatively static. The exhaust system may move in three dimensions relative to the output end of the engine. Therefore, a rigid connection between the engine and the exhaust system would put stresses on the engine and the exhaust system.
Several problems arise when trying to mate the exhaust system to the engine and provide for both axial and radial movement in the joint. The problems stem from the relative motion that must be accommodated, the high temperatures of the environment, and the need for an adequate seal. A first problem is leakage from seals such as a finger seal, which do not adequately seal the exhaust gases. A second problem is the large diameter of the seal when trying to use a labyrinth joint or rope seal that provides for sufficient radial movement. A third problem is the weight of the seal if a complex arrangement is used to accommodate the movement, but still provide adequate sealing. A fourth problem is the maintenance of the seal; longer service periods are needed and a passive failure is desired.
Although there have been significant developments in the area of sealing exhaust systems to turbine engines, considerable shortcomings remain.